This invention relates broadly to valves and fluid flow-control members, and more particularly to plug valves. More particularly still, the invention relates to plug valves which are fully lined with corrosion-resistant material, for example, the fluorocarbon polymer known as polytetrafluoroethylene (PTFE).
The problems and difficulties associated with conduit systems used to convey highly corrosive chemicals and like fluids have for many years proved to be extremely troublesome to those working in this field, and this is particularly true with respect to valves used in such an environment. Many approaches have been suggested in the past for overcoming these problems, or at least for diminishing them. Basically, it is fair to say that the principal effort embodied in these approaches has been to line the interior of the various conduit system parts, including the conduit itself, in-line fittings, and even the valves used in the system, with some non-corrosive or corrosion-resistant material.
For example, lining a valve body for such a purpose was suggested at least as long as 40 years ago (U.S. Pat. No. 1,827,266), and many different forms of this approach have since been suggested. For the most part, development has taken place through suggested uses of plastic resins for lining materials, since such materials are substantially inert chemically and, additionally, many have lubricous surface characteristics which greatly facilitate smooth and easy operation of tightly-fitting valve members.
In relatively recent times, polytetrafluoroethylene has emerged as the most favored material for forming such linings, due to its various superior qualities which are now relatively well-known. Very often, a preformed tube or other such element made of PTFE is inserted or otherwise mechanically attached in place over a given surface which is to be protected in order to form the desired lining, although other processes are also known. Valves and valve components, as well as conduit fittings and the like, using such material as a lining or coating have heretofore been described or suggested, for example, in the United States Pat. Nos. to Johnson (3,073,336), Chu (3,148,896), McFarland (3,205,113), Boteler (3,206,530), Keen (3,223,763), Yost (3,227,174), Lowrey (3,334,650), Schenck (3,438,388) and Schenck et al. (3,459,213). In certain of these prior patents, the concept is advanced of using granular or powdered PTFE for forming the lining member, in which process the powdered material is compressed into a preform and subsequently sintered so as to coalesce it into a homogeneous lining or coating which is said to be of substantially uniform density and of substantially completely fluid-impervious characteristics. This basic technique of molding powdered materials has long been used in ceramics and metallurgy, and such technique has also been long known as a way of utilizing powdered PTFE, even prior to the usage of such material in valves or fittings. For example, reference is made to the United States patents to Benning et al. (2,400,094) and Cresap (2,929,109), as well as to the Canadian Patent to Deakin (566,811).
Notwithstanding the foregoing, the total successfulness of fully-lined valves continues to remain considerably less than that which is needed and desired. Principally, the reason for this is that known types of PTFE liners are not sufficiently reliable and are subject to leakage of the corrosive fluids, as by major rupture under heat and pressure, or through the development of pinholes and other minor voids or imperfections or unduly thin sections, through which the corrosive fluid may leak or pass by permeation. This remains true despite careful manufacturing procedures and, to a certain extent, is believed to be somewhat inherent in the manufacturing processes practiced heretofore, which are themselves somewhat involved, complicated, and subject to a relatively high degree of failure or reject parts.